Open rate is the headline engagement metric for email marketing. It is calculated as (unique opens ÷ successful deliveries) × 100, where a successful delivery is any email that did not hard-bounce.
Open rate has become less trustworthy since Apple Mail Privacy Protection rolled out in 2021. Apple pre-fetches images for opted-in users, which inflates Apple-Mail opens regardless of whether the user actually read the email. Many platforms now report 'machine opens' separately from 'human opens', or de-emphasise open rate in favour of click-through rate and conversions.
Healthy benchmarks vary by industry — most senders see 15–30% with smaller, engaged lists trending higher. A sudden drop usually points to deliverability rather than content: check spam-folder placement, sender reputation, and authentication (SPF, DKIM, DMARC) before rewriting subject lines.